A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. A lithographic apparatus comprises a radiation system configured to supply a beam of radiation and to illuminate the patterning device; the apparatus further comprises a projection system configured to image said pattern onto the target portion.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. The radiation system generally includes an illumination system. The illumination system receives radiation from a source, such as a laser, and produces an illumination beam for illuminating an object, such as the patterning device (e.g. a mask on a mask table). Within a typical illumination system, the beam is shaped and controlled such that at a pupil plane of the illumination system the beam has a desired spatial intensity distribution. Such a spatial intensity distribution at the pupil plane effectively acts as a virtual radiation source for producing the illumination beam. Various shapes of said intensity distribution, consisting of (substantially uniform) light areas on a dark background, can be used. Any such shape will be referred to hereinafter as an “illumination mode”. Known illumination modes include: conventional (a top-hat shaped intensity distribution in said pupil), annular, dipole, quadrupole and more complex shaped arrangements of the illumination pupil intensity distribution. A lateral position in said pupil plane corresponds to an angle of incidence at the patterning device, and any such angle of incidence is commonly expressed as a fraction sigma (σ) of a numerical aperture NA of the projection system. Therefore, a more complete characterization of the intensity distribution in a pupil of the illumination system involves, besides an indication of the illumination mode, also an indication of parameters of the illumination mode, such as, for example, σ and NA. A combination of an illumination mode and corresponding parameters of said illumination mode is referred to hereinafter as an “illumination setting”. Known illumination settings include: a “conventional” illumination setting (whereby the intensity distribution in an illumination pupil is substantially uniform up to a certain radius defined by a parameter value of σ, where 0<σ<1, and a parameter value of the numerical aperture NA of the projection system), an annular setting (its definition comprising illumination mode parameters σinner and σouter), a dipole setting, a quadrupole setting and more complex arrangements. Illumination settings may be formed in various ways. The σ value of a conventional illumination mode can be controlled using a zoom lens while σinner and σouter values of an annular mode can be controlled using a zoom-axicon. The NA value can be controlled using a settable iris diaphragm in the projection system.
More complex settings (such as said dipole and quadrupole modes) may be formed using a diaphragm with appropriate apertures in the pupil plane or by a diffractive optical element. Typically, said diffractive optical element is arranged to generate a preselected angular intensity distribution upstream of a pupil plane of the illumination system. This angular intensity distribution is transformed into a corresponding spatial intensity distribution in the pupil plane of the illumination system.
When imaging isolated features such as lines and contact holes, the achievable depth of focus (DOF) is generally small and therefore various techniques for improving DOF when imaging such features have been developed. For isolated lines, various forms of assist features have been proposed but the size of assist features must be limited to avoid the printing of unwanted residues. Hence the extent to which such features can improve DOF is limited. For contact holes, the use of attenuated phase shift masks and three tone masks has been proposed but these methods still do not result in a particularly large DOF.
“Innovative Imaging of Ultra-fine Line without Using Any Strong RET” by Shuji Nakao et al, Optical Microlithography XIV Proceedings of SPIE Vol 4346 (2001) discloses a technique in which a sub-100 nm line pattern is imaged in two exposures. First the dark line is imaged between two bright lines then a second exposure erases the unwanted dark lines between the pairs of bright lines so that the dark lines are isolated in a bright field. Conventional illumination is used so that other large structures can be imaged at the same time.